1. Field of the Invention
The present invention relates to a trimmer capacitor and more particularly to a trimmer capacitor received in an insulating case and having no rotative shaft.
2. Description of the Prior Art
FIG. 1 is a sectional view showing an example of a conventional trimmer capacitor. The trimmer capacitor shown therein includes a stator 110 as a fixed part and a rotor 120 as a movable part. The stator 110, which is made of insulating material, such as resin, is provided on the upper surface thereof with a stator electrode 111 of, e.g., a sectorial pattern, and a stator terminal 112 is provided which extends from the stator electrode. The stator electrode 111 and stator terminal 112 are made of an electrically conductive unitary member. The rotor 120 includes a dielectric 122 which is made of, e.g., a ceramic material, and an inner electrode 121 of, e.g., a sectorial pattern embedded therein. Preferably, in order to prevent warp incidental to the firing of the dielectric 122 made of ceramic material, an idle electrode 123 may be formed in a portion where the inner electrode 121 is not formed. The stator 110 and rotor 120 are formed with central holes 113 and 124, respectively. Inserted in the central holes 113 and 124 is the stem portion of a central shaft 130 made of an electrically conductive material. The inner peripheral surface of the central hole 124 of the rotor 120 is formed with a metallized portion 142 made of silver or the like which is in electrically conductive contact with the inner peripheral edge of the inner electrode 121, and solder 143 is applied to a space defined between the metallized portion 142 and the stem portion 133, thus fixing the rotor 120 to the central shaft 130 and establishing the electrically conductive relation between the inner electrode 121 and the central shaft 130. The upper surface of the head 131 of the central shaft 130 is formed with an adjusting groove 132 for receiving the tip of a screwdriver (not shown) for adjustment of the capacitance of the capacitor. The end of the stem portion 133 of the central shaft 130 is crimped as at 134 after having fitted thereon a spring member 141 terminating at one end thereof in a rotor terminal 140. The spring member 141 functions to urge the rotor 120 against the stator 110 under suitable pressure and to produce torque.
In this trimmer capacitor, the stator electrode 111 is led out by the stator terminal 112, while the inner electrode 121 is led out by the rotor terminal 140 through the metallized portion 142, solder 143 and central shaft 130. A capacitance is formed between the stator electrode 111 and the inner electrode 121 and this capacitance is variable by the rotation of the central shaft 130, i.e., the rotation of the rotor 120.
The conventional trimmer capacitor as described above has the following disadvantages. First, since the crimping operation on the crimped portion 134 of the central shaft 130 requires strong force, the dielectric 122 of the rotor 120 often cracks. Further, the provision of electric conduction and mechanical fixation between the central shaft 130 and the inner electrode 121 (precisely, the metallized portion 142) requires soldering as at 143, which operation is very troublesome. Further, since the stator 110 is produced by molding resin with the stator terminal 112, the strength of the stator terminal 112 is low. Further, external force on the stator terminal 112 sometimes slightly vibrates the stator electrode 111, tending to cause variation in capacitance. Finally, assembly of the parts is difficult.